The present invention relates to an automatic fine tuning circuit, and more specifically to an automatic fine tuning circuit for use with a television receiver circuit.
In a television receiving system, there is known a detection circuit in which the video intermediate frequency signal is detected in a product detector which employs a video intermediate frequency carrier signal derived from the intermediate frequency signal itself. The derived video intermediate frequency carrier signal can be used for automatic fine tuning of the frequency of a local oscillator to maintain the intermediate frequency carrier signal at a desired frequency. In such an automatic fine tuning circuit, a signal from a tuned circuit in a limiter which produces the intermediate frequency carrier is phase shifted by 90 degrees and applied to a second tuned circuit associated with a phase detector. The intermediate frequency carrier signal is also applied to the second input of the phase detector without being phase shifted. The phase detector detects a signal which is proportional in amplitude and direction to the difference between the intermediate frequency carrier signal and the desired center frequency of the intermediate frequency carrier. This derived automatic fine tuning signal is applied to the local oscillator whose frequency is thereby changed in a direction which adjusts the intermediate frequency carrier signal to the proper frequency.
The presence of two tuned circuits coupled together by phase shifter capacitors produces significant interaction between the two tuned circuits. Consequently, when initially aligning the system, a repetitive, interactive, adjustment of the two tuned circuits is necessary. Furthermore, the necessity for employing a second tuned circuit for the automatic fine tuning function adds to the cost of production.